The liner of a missile motor consists of a thin layer of a polymerizable material which serves as a bond between the propellant and insulation. It is most generally formulated to be similar to the binder of the propellant. The liner has proven necessary because the insulations, in use in rocket motors, have produced too weak a bond to the propellant, and if a separation were to take place between the propellant and insulation, a burn-around would occur producing a major increase in burning surface and overpressuring of the motor.
Compounding a liner is complicated. Its composition is generally related to the composition of the propellant. It has the same binder, crosslinking agent, etc. The major differences between liner and propellant are that the liner does not contain an oxidizer (ammonium perchlorate), aluminum or nitroamino explosive compounds. The liner is applied using a "sling" lining apparatus that applies the liner through centrifugal force from a spinning spray head. A desired viscosity and certain flow properties are required of the liner. These properties are achieved through the use of inert fillers having high specific surfaces, such as a carbon black, titanium dioxide, silicon dioxide, etc.
The description of a state-of-the-art liner process which has been used in the fabrication of the Pershing II reveals the complexities of a typical liner process. Installation of the Pershing liner consists of the following, and requires approximately 40 hours to complete the following step of this state-of-the-art process:
(a) The surface of the internal insulator is degreased; PA1 (b) The insulator's surface is buffed, and again degreased; PA1 (c) A primer coat is sprayed onto the buffed insulator's surface; PA1 (d) A barrier coat is applied; PA1 (e) The liner is sprayed onto the barrier coat; PA1 (f) The liner is allowed to undergo polymerization to the B-stage; and, PA1 (g) The propellant is cast onto the liner.
It is recognized that the cost to complete the above process steps is significant, and simplication or elimination of the process steps would offer additional advantages.
The elimination of the need for a conventional-type liner provides a major cost advantage in addition to the following advantages: no processing difficulties, long shelf-life, strong chemical bonding between propellant and insulation, low off-gassing, compatibility with propellants and insulations of different composition and simple application procedure.